The present invention relates to an improved hydrofoil device whereby lift is generated primarily from a local pressure increase above that of the local free stream or ambient pressure. It relates particularly, although not exclusively, to improved hydrofoil devices with marine applications such as hydrofoil wings, propeller blades and control surfaces such as rudders and hydroplanes.
The term xe2x80x9chydrofoilxe2x80x9d as employed throughout the specification is used to describe a physical structure used to generate lift when passed through water in much the same way the word xe2x80x9caerofoilxe2x80x9d is used to describe a similar structure when employed in air. Hydrofoils may be used for dynamic support as for a wing, for a control surface such as a rudder, for propulsion as for a screw propeller and in pumps and water turbine machinery. The principles of operation of hydrofoils are however similar in each application.
Like aerofoils, hydrofoils generate lift by changing the velocity of the fluid around the surface. A change in the velocity distribution over the hydrofoil is associated with a pressure change over the surface of the hydrofoil. It is this pressure distribution, that when integrated over the surface of the hydrofoil results in force. The integral of pressure over the surface of the hydrofoil resolved normal to the flow is the lift force of the hydrofoil and the integral of pressure resolved in the direction of flow is the drag. The ratio of lift to drag is a measure of the hydrofoil""s efficiency. The lift vector may be aligned in any direction according to the application. It may be upward, downward, horizontal or at some other orientation. For convenience of further description the lift vector will normally be assumed to be upward.
For conventional hydrofoil sections as used for most shipsxe2x80x2 propellers the acceleration of the fluid around the hydrofoil implies an associated reduction in pressure. It is hence the reduction in pressure over the hydrofoil that contributes most to lift.
The limit in the application of such devices to high inflow velocities lies in the magnitude of the pressure reduction possible resulting from velocity increase. When the local pressure falls below the vapour pressure of the water, the water vaporises and a vapour pocket forms. The forming of such vapour pockets is often referred to as cavitation. Once the water vaporises, the pressure cannot be significantly reduced further and so this condition represents a lower limit of pressure and a hence a limit to the force capability of a hydrofoil.
The present invention relates to an improved hydrofoil device whereby lift is generated primarily from a local pressure increase. This approach represents a significant departure from conventional hydrofoil design where lift is primarily generated by means of pressure reduction, or suction. An object of the present invention is, amongst other benefits, to avoid the above mentioned limitation on force capability imposed by the onset of cavitation, thereby enabling application with high inflow velocities that would otherwise result in, or be limited in for by, cavitation. The device however remains effective at reduced inflow velocities.
It is generally known that a decrease in fluid velocity is accompanied by a pressure increase. This phenomenon is widely exploited in aviation in the form of a spoiler used to intercept the flow over a wing thereby reducing velocity and effecting a local pressure increase. The use of such a device is however accompanied by a flow separation and the formation of low pressure region behind the device. It is the pressure drag associated with this low pressure region that contributes to such devices poor efficiency.
The application of similar devices to the marine environment is further complicated by the possibility of multiple phase flows, namely liquid and gas While a pressure increase ahead of the device may be possible, the separated region formed behind the device may be of sufficiently low pressure to initiate cavitation.
Australian patent application No. AU 43617/96, which is the Australian National Phase of PCT/SE95/01582, describes a mechanism for dynamic trimming of the floating position of a planing or semi-planing ship hull so as to counteract the pitching and rolling movements in operation. The mechanism includes a plate that is mounted submerged transversely to the relative water flow and protrudes at the lower end of the stem of the ship hull for generating a vortex having an upwardly and forwardly directed velocity component in front of the plate. This vortex creates a water volume below the stem of the ship hull that has an increased pressured directed to the bottom surface of the hull and which therefore directly affects the trimming position of the ship at prescribed velocities. This mechanism is however limited in application to transom stem vessels where the region behind the transverse plate can be ventilated to minimise pressure drag.
The present invention was developed with a view to providing a means to exploit the benefits of a local pressure increase on a surface of a hydrofoil whilst reducing the detrimental influence of pressure drag.
Throughout this specification the term xe2x80x9ccomprisingxe2x80x9d is used inclusively, in the sense that there may be other features and/or steps included in the invention not expressly defined or comprehended in the features or the steps specifically defined or described. What such other features and/or steps will include will be apparent from the specification read as a whole.
According to one aspect of the present invention there is provided an improved hydrofoil device, the hydrofoil device comprising:
a first surface and a second surface located on an opposite side of the device to said first surface, wherein water flows over the first and second surfaces from a leading edge to a trailing edge of the device; and,
a protrusion provided adjacent the trailing edge of the device so as to produce a surface discontinuity on said first surface wherein said protrusion is arranged with an included angle to the upstream direction of the blade of less than or equal to 90xc2x0 whereby, in use, the flow of water over the surface is deflected back on itself, a resulting increase in the local pressure on said first surface ahead of said protrusion contributes substantially to the lift generated by the hydrofoil device.
Advantageously said protrusion is provided by means of an adjustable interceptor, said interceptor having a moveable surface forming said included angle whereby, in use, the extent to which said moveable surface protrudes from said first surface can be varied to control the degree of lift generated by the hydrofoil device. Preferably said moveable surface can be made to protrude from either one of said first surface or said second surface so as to change the direction of lift generated by the hydrofoil device.
Typically said hydrofoil device further comprises means for ventilating a separated cavity formed adjacent a trailing edge of the device bounded by the water flowing over said first and second surfaces. Preferably said means for ventilating comprises a flow path for air from atmosphere whereby, in use, air will be drawn into the cavity by natural ventilation.
In one embodiment said adjustable interceptor is formed by a trailing edge section of the hydrofoil device, said trailing edge section being pivotally mounted on a main body of the device by means of one or more pivot plates. Preferably each said pivot plate extends outwards to a maximum depth of said protrusion and extends upstream at least twice this distance. Typically each said pivot plate is pivotally coupled to the interior of said main body by a pivot pin. Advantageously one of said pivot plates is located at an end of a span of the hydrofoil device to prevent pressure loss at a tip of the device.
In another embodiment said adjustable interceptor is formed by a moveable section slidably mounted in guides so as to be moveable in a direction substantially perpendicular to the direction of flow of water around said hydrofoil device. Advantageously said guides are formed by guide plates fixed to a main body of the device and adapted to minimise spilling of the localised high pressure region across said first surface.
According to another aspect of the present invention there is provided an improved propeller for marine propulsion, the propeller having a plurality of hydrofoil blades, and at least one of the hydrofoil blades comprising:
a first surface and a second surface located on an opposite side of the blade to said first surface, wherein water flows over the first and second surfaces from a leading edge to a trailing edge of the blade; and,
a protrusion provided adjacent the trailing edge of the blade so as to produce a surface discontinuity on said first surface and wherein said protrusion is arranged with an included angle to the upstream direction of the hydrofoil of less than or equal to 90xc2x0 whereby, in use, as the flow of water over the surface is deflected back on itself, a resulting increase in the local pressure on said first surface ahead of said protrusion contributes substantially to the thrust generated by the propeller.
Preferably said protrusion is provided by means of an adjustable interceptor, said interceptor having a moveable surface forming an included angle to the upstream direction of the blade whereby, in use, the extent to which said moveable surface protrudes from said first surface can be varied to control the thrust generated by the propeller.
Preferably said moveable surface is formed on a slidable plate which is supported in diagonal guide slots oriented in a main body of the blade so that actuation from a hub of the propeller of said slidable plate is orthogonal to the direction of the blade movement.
Advantageously said hydrofoil device further comprises:
a second protrusion provided adjacent the leading edge of the device so as to produce a surface discontinuity on said second surface whereby, in use, a resulting flow separation defined on said second surface reduces friction and contributes substantially to stable flow conditions across said second surface.
Preferably means for ventilating the second surface behind said second protrusion are provided so as to prevent reattachment of the separated flow and inhibit the possibility of cavitation on said second surface.
Preferably said second protrusion is adjustable between a first position in which a leading edge surface discontinuity is formed on said second surface and a second position on which a leading edge surface discontinuity is formed on said first surface.
Typically said second protrusion is formed by a leading edge portion of the hydrofoil device which is moveable between said first and second positions.